Lesson focus: A unicellularMade of just one cell. organism performs all essential life processes within one cell.
Compare a bacteriumA tiny living thing made of just one simple cell, without a nucleus. with a unicellular eukaryoteA living thing whose cells have a nucleus, such as an animal, plant or fungus. and explain exchange limits.
Build on earlier learning
Movement of substancesUse diffusion and surface-area-to-volume reasoning to explain single-cell exchange.
Practical
Looking at pond-water organisms
- Place a drop of pond water on a slide and lower a coverslip at an angle to avoid air bubbles.
- Search at low power, then increase magnification.
- Look for movement, colour (chloroplasts) and cell shape.
Record: Drawings of two organisms with notes on how each moves and feeds.
Safety: Wash hands after handling pond water.
You will be successful when you can…
- State that bacteria are cells without a nucleus.
- Identify eukaryotic nucleus and membrane structures.
- Explain direct exchange using size and SA:V.
- Predict why larger organisms need specialised systems.
Words that carry the science
- unicellular
- Made from one cell.
- bacterium
- Single-celled prokaryote whose DNA is not enclosed in a nucleus.
- eukaryote
- Organism whose cells contain a nucleus.
- prokaryote
- Organism whose cells lack a nucleus and membrane-bound organelles.
- division of labour
- Different cells or structures specialising in different functions.
Fit a whole life into one cell
01One cell carries every essential process
A unicellularMade of just one cell. organism takes in materials, respires, excretes, grows, responds and reproduces within one cell. It may still contain specialised internal structures.
A bacteriumA tiny living thing made of just one simple cell, without a nucleus. has a membrane, cytoplasm, ribosomes and DNA not enclosed in a nucleus.
02Not every single cell is a bacterium
Amoeba and Euglena are unicellularMade of just one cell. eukaryoteA living thing whose cells have a nucleus, such as an animal, plant or fungus. with nuclei and membrane-bound compartments. Euglena may photosynthesise; an amoeba changes shape and engulfs food.
03Small size supports exchange
A high surface-area-to-volume ratio and short internal distances allow many unicellular organisms to exchange materials directly with surroundings. In simple words: a single-celled organism is tiny. It has a lot of outside (cell membrane) compared with its inside, so enough substances can get in and out through its surface. And nothing inside it is far from the outside, so substances only travel a short distance.
As size rises, ratio falls and diffusion paths lengthen. Multicellularity enables division of labourDifferent cells or body parts doing different jobs, so the whole organism works well. and transport between specialised regions.
Amoeba and euglena on the move
Single-celled organisms do every life process in one cell. An amoeba flows towards food with pseudopodia and engulfs it in a food vacuole. A euglena swims with its flagellum, senses light with its eyespot and makes food in its chloroplasts.
Purple: nucleus. Yellow: food (bacteria), then food vacuoles inside the cell.
Controls
Each organism is drawn to its own scale bar. Speeds are illustrative.
Infer from a diagram
A cell has a membrane, cytoplasm, ribosomes and a loop of DNA but no nucleus.
- Identify cellular evidence: membrane and cytoplasm.
- Locate genetic material: DNA is present.
- Note that the DNA is not enclosed in a nucleus.
- Classify it as a prokaryotic cell, consistent with a bacteriumA tiny living thing made of just one simple cell, without a nucleus..
How to make the reasoning visible: Classification uses the arrangement of structures, not just cell size.
Single-cell comparison
Use the evidence supplied; drawings are not to the same scale.
| Feature | Bacterium | Amoeba-like eukaryote | Meaning |
|---|---|---|---|
| Nucleus | Absent | Present | Different cell organisation |
| Cell membrane | Present | Present | Controls exchange |
| Ribosomes | Present | Present | Protein synthesis |
| Typical size | Smaller | Larger | Scale must be calibrated |
Why is “bacteria have no DNA” false?
Why does a larger unicellular organism face an exchange challenge?
Use the table. Give one way a bacterium and an amoeba are similar and one way they are different.
Check your understanding
Answer from memory, then check against the suggested answer.
What does unicellular mean?
Do bacteria contain DNA?
What is division of labour?
Name one unicellular organism.
What is a prokaryote?
A pond cell has a nucleus and chloroplasts. Is it a bacterium?
Why can a multicellular organism become larger than most unicellular organisms?
Euglena can make its own food in the light but also take in food. Is it more like a plant or an animal? Explain.
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Single-celled organisms take in substances straight from their surroundings. Answer the parts to explain how. (The surface area is the outside of the cell. The volume is the inside.)
How big are single-celled organisms?
(1)How much surface area does a tiny cell have, compared with its volume?
(1)Where do substances get into the cell?
(1)How far do substances travel once inside?
(1)Total for this question: 4 marks
Command word: Use both ideas: the surface and the short distance.